The present invention relates to an image display device and a method for manufacturing the same. In particular, the invention relates to an image display device, also called a flat panel display of emissive type, using thin-film type electron source array.
A type of image display device has been developed, which uses emission type electron sources, also called thin-film type electron sources, in micro-size and of integratable type. The thin-film type electron source is designed in a three-layer thin-film structure comprising a top electrode, an electron accelerator, and a bottom electrode. Voltage is applied between the top electrode and the bottom electrode, and electrons are emitted from the surface of the top electrode into vacuum space. There are different types of devices, which include, for instance: an MIM (metal-insulator-metal) type with a metal layer, an insulator and a metal layer laminated on each other, an MIS (metal-insulator-semiconductor) type with a metal layer, an insulator, and a semiconductor layer laminated on each other, a metal-insulator-semiconductor type, an EL type, a porous silicon type, etc.
The Patented References 1 and 2 describe an MIM type. The Non-Patented Reference 1 describes a metal-insulator-semiconductor type. The Non-Patented Reference 2 describes a metal-insulator-semiconductor-metal type. The Non-Patented Reference 3 describes an EL type, and the Non-Patented Reference 4 discloses a porous silicon type.
FIG. 1 is a cross-sectional view to explain an example of structure of a thin-film type electron source by taking an example of the MIM type. FIG. 2 is a drawing to explain operating principle of the thin-film type electron source. The MIM type thin-film electron source comprises a bottom electrode 11 formed on a substrate 10, a top electrode 13 laminated to intersect the bottom electrode via a tunneling insulator (also called a tunneling insulator) 12, and an interlayer insulator 14. Electric current is supplied to the top electrode 13 via a top electrode bus line 16 and a contact electrode 15.
Now, description will be given on operating principle of the thin-film type electron source shown in FIG. 1 by referring to FIG. 2. In FIG. 2, driving voltage Vd is applied between the top electrode 13 and the bottom electrode 11, and electric field within the tunneling insulator 12, serving as an electron accelerator, is turned to about 1 to 10 MV/cm. Then, electrons near Fermi level in the bottom electrode 11 pass through potential barrier due to tunneling phenomena. Electrons are injected to the tunneling insulator 12 and to a conduction band of the top electrode 13, and the electrons are turned to hot electrons.
These hot electrons are diffused in the tunneling insulator 12 and in the top electrode 13 and lose energy. A part of the hot electrons having energy higher than the work function φ of the top electrode 13 are emitted into vacuum space 20. In other types of the thin-film electron sources, the principle may be somewhat different but there are common features that hot electrons are emitted via the thin top electrode 13.
The bottom electrode comprising the thin-film type electron sources, the top electrode arranged to intersect the bottom electrode, and a top electrode bus line for supplying electric current to the top electrode are placed in form of a 2-dimensional matrix to make up a thin-film type electron source array. Then, a display signal is applied to the bottom electrode, and a scan signal is applied to the top electrode (top electrode bus line), and electrons from the thin-type electron source on the intersections are directed toward phosphor and are excited. As a result, an image display device is made up. In this case, the top electrode but line is turned to scan line bus line. The thin-film type electron sources are described, for example, in the following references:                [Patented Reference 1] JP-A-7-65710        [Patented Reference 2] JP-A-10-153979        [Non-Patented Reference 1] J. Vac. Sci. Technol.; B11(2), pp. 429-432 (1993).        [Non-Patented Reference 2] Jpn. J. Appl. Phys.; Vol. 36, p. 939.        [Non-Patented Reference 3] Jpn. J. Appl. Phys.; Vol. 63, No. 6, p. 592.        [Non-Patented Reference 4] Jpn. J. Appl. Phys.; Vol. 66, No. 5, p. 437.        
As described above, in this type of image display device, a display signal is applied on the bottom electrode, and a scan signal is applied on the top electrode (top electrode bus line), and thin-film type electron sources at intersections are selected. For this reason, insulation between the bottom electrode of the thin-film type electron source array and the top electrode (top electrode bus line) is very important. If insulation between the two electrodes is poor, electric short-circuiting may occur between the bottom electrode and the top electrode or the top electrode bus line, and this may cause defects in the image. In this respect, the tunneling insulator, serving as an electron accelerator, and the interlayer insulator for limiting the electron emission region must be free of defects.
Conventionally, an electrochemical film deposition method called anodic oxidation has been used for forming a tunneling insulator and an interlayer insulator. This film deposition method is extremely superior to the other film deposition method in providing uniform and even film quality and film thickness, and it is suitable for the formation of a display panel, which makes up a large scale (large area) image display device comprising this type of electron source array. However, in the anodic oxidation, there are the problems as described in (1) to (3) below.
(1) When there is a portion where electric current does not flow due to foreign object or the like attached on the surface, poor insulation occurs. (2) In case a display panel with the thin-film type electron source array formed on it is provided, mechanical damage may occur on the interlayer insulator of the thin-film type electron source array because of the atmospheric pressure applied on the cathode substrate via spacers, and dielectric breakdown of time zero is induced. (3) In general, capacitance of the thin-film type electron source is higher than that of a liquid crystal device. This is because specific dielectric constant of alumina, serving as the insulating film, is as high as 10, and also because film thickness is as thin as about 10 nm. For this reason, a driving circuit chip (IC or LSI) having sufficient ability to supply electric current must be used, and this implies that higher circuit cost compared with the liquid crystal device is required.
In the capacitance, the tunneling insulator and the interlayer insulator occupy one half of the capacitance respectively. The tunneling insulator is about 1/10 in film thickness and area compared with the interlayer insulator. On the other hand, dielectric constant is the same for these two (specific dielectric constant: up to 10), and capacitance is almost the same. To decrease the parasitic capacitance, film thickness of the interlayer insulator should be increased, while it is difficult to simply increase the oxidation voltage because of the relation with withstand voltage of the resist mask for local oxidation.
It is an object of the present invention to provide an image display device, which is free of display defects and has long service life, and by which it is possible to prevent dielectric breakdown between the bottom electrode comprising the thin-film type electron sources and the top electrode (top electrode bus line).
The electron sources as described above are arranged in form of matrix in a plurality of columns (e.g. in horizontal direction) and in a plurality of rows (e.g. in vertical direction). A number of phosphor film (phosphors) and anodes to match each of the electron sources are arranged in vacuum space to make up an image display device. When an image is displayed on the image display device with the arrangement as described above, a driving method called “line-sequential driving” is normally adopted. This is a method, by which the displaying for each frame is performed for each scan line (horizontal direction) when 60 still pictures (60 frames) are displayed per second. Therefore, the electron sources to match the number of data lines positioned on the same scan line are all operated at the same time. During operation, electric current flows on the scan line. The value of this electric current is a product of the value of the electric current (consumed by the electron source contained in a sub-pixel (a mono-color pixel to make up one pixel for full color display)) multiplied by the number of all data lines. This scan line current causes voltage drop along the scan line due to wiring resistance, and this hinders uniform operation of the electron source. In particular, when it is wanted to have a large size image display device, the voltage drop caused by the wiring resistance of the scan line is a serious problem.
In the image display device using the thin-film type electron source as the cathode (electron source), the capacity between the scan line and the data line must be reduced for the purpose of reducing power consumption based on the decrease of charge-discharge of the power to the cathode, of decreasing the load of the driving circuit (driver) and of preventing signal delay caused by the decrease of CR time constant. To attain these purposes, the most promising approach is to use a coating type insulating film, which has lower dielectric constant and is easier to have higher film thickness as the interlayer insulator between the scan line and the data line.
However, the coating type insulator is shrunk due to drying and firing after coating, and it is less resistant to tensile stress. In contrast, in the metal wiring formed on the interlayer insulator, a material with relatively higher tensile stress is used. As a result, when a metal wiring having higher tensile stress is deposited on the interlayer insulator, which is formed by the coating type insulating film, cracking may occur, and it is easily peeled off, and this leads to one of the causes for disconnection.
As the material of the wiring, chromium (Cr) or aluminum (Al) is widely used. Chromium is a metal material with very high tensile stress, and when it is deposited on the coating type insulating film, cracking is very likely to develop. Also, the film of the alloy produced from aluminum added with neodymium (Nd) or tantalum (Ta) has relatively low tensile stress immediately after it is deposited as film, while, in the process of sealing and heating of the display substrate, the stress of the alloy film is changed and strong tensile stress develops, and cracking occurs. As a result, the trouble such as disconnection occurs, and this leads to lower reliability. This is not limited to the image display device of matrix type, in which data lines and scan lines are separately formed on the interlayer insulator, but it is the same in other film structures of similar type, which comprise metal thin-film on the coating type insulating film.
It is another object of the present invention to provide an image display device with high reliability and with metal film structure not to cause cracking even when it is deposited on the coating type insulating film.
The image display device according to the present invention is provided with a vacuum panel container, which comprises a cathode substrate where a plurality of thin-film type electron sources are arranged with predetermined spacing, an anode substrate where spot-like or linear phosphor films are arranged to face to each other, a plurality of spacers for supporting said cathode substrate and said anode substrate with predetermined spacing, and a frame glass for maintaining vacuum condition, wherein there are provided a plurality of electric bus lines extending in row direction and in column direction crossing perpendicularly via interlayer insulators, and said cold cathode type electron sources are connected with said electric bus lines in column direction and in row direction at positions corresponding to each of intersection coordinates, and image display is performed by line-sequential driving of the cold cathode type electron sources.
The thin-film type electron sources are provided with a bottom electrode, a top electrode and an electron accelerator interposed between these two. As the interlayer insulator, a thin-film lamination of at least two layers is used, which contains a thin-film insulating material formed by the coating film and having specific dielectric constant of not more than 5 and a thin-film insulating film produced by vacuum film deposition.
Also, in the present invention, in addition to said interlayer insulator of the thin-film lamination with at least two layers, bus lines connected to the top electrode among a plurality of bus lines or the insulating film formed by vacuum deposition among the interlayer insulator are arranged in such manner that at least a part of the pattern end portion of the insulating material suitable for coating film deposition can be covered.
Further, in the present invention, the bottom electrode is made of aluminum or aluminum alloy, and the electron accelerator can be designed as an anodic oxidized film. Also, inorganic or organic polysilazane or a mixture of these two types of polysilazane may be used as the material of the insulation layer formed by coating.
Also, according to the present invention, metal films (such as bus lines on the coating type insulating layer when the coating type insulating film is used as the interlayer insulator) are deposited by using aluminum or copper. Further, on the coating type insulating film, a metal film in contact with the coating type insulating film is made of a high melting point metal such as chromium (Cr), molybdenum (Mo), nickel (Ni), tungsten (W), etc. The metal film is formed in thickness of not more than 10 nm, and a laminated wiring with aluminum or copper deposited on it is used.
More concrete description will be given below on the image display device.
Specifically, the invention provides an image display device, which comprises a cathode substrate and a phosphor substrate, wherein said cathode substrate has thin-film type electron sources arranged in form of matrix, said electron sources are electrically connected with the scan lines and the data lines and emit electrons at each of intersections of a plurality of scan lines and data lines, said phosphor substrate contains phosphor layers with a plurality of colors arranged to match each of said electron sources, there is further provided a coating type insulating film formed by coating method as an interlayer insulator to provide insulation between the scan lines and the data lines, a metal film with internal stress of −200 MPa or lower is used as the scan lines or the data lines formed in contact with the coating type insulating film.
The metal film to prepare the scan lines or the data lines is made of aluminum (pure aluminum) or copper (pure copper) not containing additional metal, and a high melting point metal with melting point of 1200° C. or more is laminated on the metal film. As the high melting point metal, at least two types selected from chromium, nickel, molybdenum, and tungsten, or alloys of two types of more of these metals are suitable.
Also, in the image display device of the present invention, a high melting point metal film with film thickness of not more than 10 nm is formed on the coating type insulating film as scan lines or data lines to be formed on the coating type insulating layer. A metal film of pure aluminum or pure copper is deposited on it, and a laminated film is prepared by sequentially forming thin films of metal materials with melting point higher than that of pure aluminum or pure copper on it.
As the coating type insulating film, an insulating film prepared by dry process under vacuum condition or an insulating film prepared by wet process in solution or organic or inorganic silicon polymer or polysilazane formed by coating method on the laminated films is used.
According to the present invention, an image display device can be provided, which can prevent initial dielectric breakdown (time zero) and to improve production yield. Dielectric breakdown over time can be prevented and perfect operation and longer service life are assured.
Also, the present invention provides an image display device with high reliability, by which it is possible to prevent cracking and film peeling caused by stress of metal film lines formed on the coating type insulating layer even when the coating type insulating layer is used as the interlayer insulator. Because the coating type insulating layer with high film thickness and low dielectric constant is used as the interlayer insulator, it is possible to achieve lower capacity of bus lines, to decrease power consumption, to reduce driver load, and to prevent signal delay.